In-line demetallization process for flexible metallized substrates

ABSTRACT

The present invention discloses a continuos process for the partial demetallization of a first multilayer substrate, comprising at least one metallic layer  21 , characterized in that a designed lacquer comprising at least one metal dissolving etchant  25  locally reacts with said metallic layer  21  and that the dissolved metal remains within said multilayer structure and that the dissolution of the metal allows the creation of a window in said metallic layer without the necessity of a washing step and in that said partial demetallization is suitable to be carried out on standard gravure or flexo printing presses or coating equipment.

FIELD OF THE INVENTION

The present invention is related to the field of metallized packagingsubstrate needing a partially demetallized area and more particularly toa simplified process for obtaining the same.

TECHNOLOGICAL BACKGROUND

In flexible packaging applications, polymeric films and/or paper websare often combined to a metallic layer generally consisting ofaluminium. This metallic layer can be a self-supporting foil, typicallybetween 6 and 15 μm thick, or it can be a much thinner layer, generallybelow 0,1 μm thick, on a polymeric or paper support. This metallic layeris usually applied by a vacuum coating process, in which vaporised metalatoms adhere to a suitable substrate. This vacuum metallization processis extensively described in the literature.

Metal foils and metallic coatings have several functions, includingbarrier functions with regard to atmospheric gases, water vapour,radiation, etc. and, in addition, play an important role in themarketing aspects of a package. Such metallic layers give a particularbrilliance and colour intensity to the overlying printed design, and,where visible by themselves as a metallic design element, give aperception of quality and protection of the package contents. In manycases though, when the barrier needs of the package allow it, theproducer would wish to combine these positive marketing aspects of ametallic layer with a partial window in the metallic layer. In the caseof transparent polymeric films the main purpose would be to allow forvisual inspection of the packaged product by the consumer in the retailphase. In the case of multilayer structures involving paper or othernon-transparent substrates, there might be other functional or marketingadvantages in having a partial window in the metallic layer.

In most of the following, we focus on the case of transparent polymericfilm laminates with thin metallic coatings as being the most importantclass of multilayer materials in which the current invention could beapplied. Here the current industrial practice for obtaining a partialdemetallization has been a procedure involving the following processingsteps:

-   a) a printing step, involving a metallized film, typically    consisting of an oriented coextruded polypropylene film, between 15    and 30 μm thick and vacuum coated with a layer of aluminium, about    100 to 1000 Å thick, which is partially printed on a regular    printing line (typically a gravure or flexo press) using a suitable    ink system and an overlacquer to protect the inks during subsequent    processing. In most cases, a primer is applied between the    metallized layer and the printing inks to improve adhesion. When    this printed film is intended for partial demetallization, care is    taken that neither primers nor inks or overlacquers cover the    aluminium in the area to be demetallized. In the case that an    unprinted metallized film is intended to be partially demetallized,    only the protective overlacquer would need to be printed, possibly    with the addition of a suitable primer;-   b) a demetallization step, involving the passage of the film    prepared according to step (a) through a concentrated sodium    hydroxide (NaOH) solution in water, whereby the exposed portions of    the metallic aluminium are dissolved and the dissolved metal is    subsequently washed away with water, followed by a drying operation    to remove excess moisture;-   c) a lamination step, whereby the printed demetallized film is taken    on a laminating machine and bonded to another self-supporting film    web, typically 15-30 μm thick, using a suitable adhesive system    (most often a two-component polyurethane adhesive).

The procedure described above and in practical use today is seen toinvolve at least three separate converting steps, which makes it a verycostly process, limiting its market penetration to high-end products. Afurther disadvantage is the time loss because of the logistics of thethree-step process, especially if converting and demetallizationequipment are found in different production sites. A furtherdisadvantage is the fact that particular in-line operations, such as theapplication of a cold seal lacquer on the backside of the metallizedfilm, become impossible because of the various processing steps. Afurther disadvantage is the lack of an optimal quality control in theprinting step, since the final result only becomes visible after thedemetallization step.

State of the Art

The above multi-step procedure being the current industrial practice, webelieve that the following documents represent the closest prior art.

U.S. Pat. No. 5,628,921 describes a process for carrying out theclassical demetallization involving a caustic solution and a washingstep, in-line with a gravure printing operation, through the use of adedicated machinery custom made for this purpose and essentiallyconsisting of a classical demetallization equipment connected to aclassical gravure printing press. It would seem that this process andequipment has the advantageous possibility of in-line quality controlchecking the demetallized area in respect of the printed design, this ishowever achieved at the expense of a much higher investment cost forthis complicated machinery.

U.S. Pat. No. 3,647,508 discloses a process for carrying out thedemetallization whereby the etching agent is mixed with a film-formingdispersion thereby achieving that the etching agent can be containedwithin a dried coating remaining on the web. However this method onlyclaims particular effects on the conductivity, reflectivity and adhesionof the final product, not transparency, and an optional washing step isdescribed evidently for this purpose.

The purpose of the present invention is to obtain clarity andtransparency (high transmission and clarity and low haze) of thedemetallized window, which still requires a washing step in the priorart.

In summary, neither of the two described processes constitutes asignificant breakthrough versus the current practice described in thetechnological background.

Aims of the Invention

The present invention aims to provide a simplified process for partialdemetallization of flexible substrates, performed on standard equipmentsuch as a gravure or flexo press, rather than on machinery specificallydesigned for demetallization. Furthermore, this invention aims to reducecomplexity and cost of the entire process by performing said processin-line with other converting operations such as printing, laminatingand/or coating in one continuous operation.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a metallized film complex comprising differentcomponents according to a first embodiment of the present invention;

FIG. 2 represents a metallized film complex comprising differentcomponents according to a second embodiment of the present invention;

FIG. 3 represents a metallized film complex comprising differentcomponents according to a third embodiment of the present invention;

FIG. 4 represents a standard process machinery able to achievedemetallized film according to anyone of the embodiments of the presentinvention.

SUMMARY OF THE INVENTION

The present invention discloses a continuous process for the partialdemetallization of a first multilayer substrate, comprising at least onemetallic layer, characterised in that a designed lacquer comprising atleast one metal dissolving etchant, locally reacts with said metalliclayer and that the dissolved metal remains within said multilayerstructure and that the dissolution of the metal allows the creation of awindow in said metallic layer without the necessity of a washing stepand in that said partial demetallization is suitable to be carried outon standard gravure or flexo printing presses or coating equipment.

A possible embodiment of the present invention is that said processfurther comprises a lamination step of the partly demetallizedmultilayer support with at least one second substrate.

Furthermore, the present invention discloses that at least one of saidsubstrates is selected from the group consisting of polymeric films,paper, metallic foils and non-woven substrates.

Another possible embodiment is that at least one of said substrates istreated by at least one coating operation and/or at least one printingoperation.

The present invention also shows that said coating or printing operationis carried out on a different substrate surface than that where thedemetallization is carried out, yet involves a patterned print orcoating in register with the demetallized area and/or the other printeddesigns in or on the multilayer structure.

Another key feature of the present invention is that the demetallizationstep achieves a light transmission of at least 90% within thedemetallized area without a washing step.

Furthermore, the demetallization step to obtain a light transmission ofat least 90% is carried out on standard gravure or flexo printingpresses or coating equipment without necessitating specific dedicatedequipment for demetallization.

Another key feature of the present invention is that the etchantconcentration in the etchant lacquer substantially corresponds to thestoechiometrical amount of said etchant to dissolve the amount of metalpresent on the film.

Alternatively, the etchant concentration in the etchant lacquercorresponds to a slight excess of the stoechiometrical amount of saidetchant to dissolve the amount of metal present on the film.

Finally, the present invention discloses a multilayer support obtainableby any of the previous claims comprising windows in continuous and/ordiscontinuous supported metallic layers characterised in that saidwindows contain the total quantity of the residues resulting from thedemetallization by means of an etching product.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses a process for partial demetallization,whereby the etching agent is contained in a suitable formulated lacquerwhich can be applied onto the metallized web using commonly availablefilm converting equipment (such as a gravure or flexo press or coatingline) and said lacquer is designed to remain in contact with the web,thereby also retaining the dissolved metal in place, such that the needfor washing and drying the demetallized part of the web is eliminatedwhile simultaneously achieving optimal clarity and transparency of thedemetallized area.

The following measurements have been achieved on a suitable equipmentspecified hereunder to show the high transparency reached on samplesrealised according to the process of the present invention:

-   Equipment: Haze-Gard plus-   Measurement: according to norm ASTM-1003-   Results:    -   (a) on a demetallized laminate:        -   transmission=94.1%±1.2%        -   haze=4.7%±0.6%        -   clarity=96.1%±0.4%    -   (b) on a transparent laminate:        -   transmission=94.9±1.0%        -   haze=3.7±0.3%        -   clarity=96.2±0.3%

The results show that only negligible differences exist between thedemetallized samples and ordinary transparent laminates.

The process achieves the demonstrated transparency by a combination oftwo actions, the first being the elimination of chemical reactivity ofthe etchant versus the adhesive layer it contacts in the region of thetransparent window, by fine-tuning the amount of etchant lacquer appliedonto the metallization through choosing a suitable gravure cylinderdepth and adapting the etchant concentration in the wet etchant lacqueras needed, thereby being close to (and only slightly towards excess of)the stoechiometrical amount of etchant needed to completely dissolve theamount of metal present on the film; and a second action being theelimination of any chemical reactivity of the etchant towards the sameadhesive which could result from an interaction on the machine betweenthe etchant lacquer and the wet adhesive which would be expected toresult in a partial dissolving of the etchant lacquer into theadhesive-containing vessel on the laminating section, at which time theetchant is seen to chemically react with the adhesive.

This invention by itself means a major simplification and cost saving ofthe demetallization step, since it can now be performed on commonlyavailable equipment rather than on machinery specifically designed fordemetallization. Furthermore, this invention immediately gives rise to afurther significant reduction in complexity and cost of the entireprocess, since the demetallization step can easily be performed in-linewith other converting operations such as printing and laminating, in onecontinuous operation. This has the added advantage of allowing immediatecontrol of the demetallized result such that an adjustment in an earlierprocess step (e.g. the printing position of the protective overlacquer)can be easily made.

A further advantage is the possibility of carrying out particularoperations or applying particular products which previously could notwithstand the step of demetallization/washing/drying, or were impossiblebecause the lack of registration between the printed design and thisadditional product, an example being the application of a lacquer on theoutside of the laminate in a fixed position with regard to the printeddesign.

Description of a Preferred Embodiment of the Invention

In the first embodiment of the present invention, as represented in FIG.1, the metallized substrate 20 as defined above, is partially printedusing a suitable ink system 23, typically with the aid of a primer 22 toimprove ink adhesion on the metallization 21, and a protectiveoverlacquer 24 on the printed areas. The demetallization in theunprotected areas is achieved by applying a demetallization lacquer 25containing the etching agent onto the remaining exposed surface of themetallization. This is done in-line with the printing step, and can onsuitable printing presses be followed by an in-line laminating stepusing a suitable laminating adhesive 26 as above. When usingsolvent-based adhesives it will be advantageous to apply the adhesive tothe non-printed web so that the wet adhesive 26 and the solventscontained therein cannot affect the printing inks 23 and especially thedemetallization lacquer 25.

In a second embodiment of the present invention, represented in FIG. 2,the process could be set up so that first the demetallization lacquer 25is locally printed on the metallized layer, followed by an all-overcoated protective lacquer 24, now also covering the demetallizationlacquer, and then by the printing inks 23 where intended. Again thefinalisation of the laminating step can be done in-line. Thisalternative procedure would have the added benefit of allowing, formarketing reasons, part of the printed design not be backed by themetallic layer, thereby giving a distinctive change in appearance.

In a third embodiment of the present invention represented in FIG. 3, anink type 23 is used which resists (is not chemically affected by) theetchant 25, but is not a barrier to it, together with a metallizationprimer 22 which is a barrier to said etchant. In this embodiment theprotective overlacquer 24 is not needed. As in the second embodiment,this one allows inks to be backed by metal or by transparent film, andachieves this extra capability even while requiring less gravurepositions. If required, other converting operations remain possiblein-line.

While the invention has been illustrated and described in what areconsidered to be the most practical and preferred embodiments, it willbe recognised that many variations are possible on the positioning ofthe different layers and come within the spirit and scope thereof, theappended claims therefore being entitled to a full range of equivalents(inks can be omitted, coatings added, and generally several possiblepositions are possible for each component of the multilayer structure).Known possibilities, which are also not further explored here, includemaking a partially demetallized multilayer structure containing only oneself-supporting substrate, or alternatively three or more of suchsubstrates, as well as having a metallization layer 21 not directlysupported by a substrate but rather applied onto a coating and/orprinting ink. Furthermore, completely similar multilayer structures canbe made using paper and/or pigmented films, either metallized or not, insuch multilayer structures in which case no transparency of the totalstructure is achieved, but the optical clarity of the demetallized layeritself might be just as appreciated.

Example of a Demetallization Process According to the First Embodimentof the Present Invention

During the process, a reel of polymeric film 20, typically consisting ofbiaxially oriented polypropylene and metallized on one side with a layerof vacuum deposited aluminium 21, is placed in the unwind position 11 ofa heliogravure press with in-line laminating capability. The film runsthrough consecutive gravure printing stations 1 to 6 of the machine, andundergoes the following consecutive operations:

-   a) in gravure station 1 the entire portion of the metallization    layer 21 which is intended to remain on the final material, is    coated with an adhesion-promoting primer 22,-   b) in stations 2, 3 and 4 the individual colours of the printing    design 23 are printed on the film,-   c) in station 5 the printed area 23 is covered by a protective    overcoating 24,-   d) in station 6 the remaining portions of uncovered metallization 21    are covered with the demetallization lacquer 25. As the intended    chemical reaction takes place, the part of the metallized layer 21    in contact with the demetallization lacquer 25 becomes transparent.    From unwind position 12, a second reel of film 27 is unwound,    typically consisting of a transparent biaxially oriented    polypropylene, and passes through gravure station 7 in which a layer    of adhesive 26 is applied to the inside surface of the film, after    that, the adhesive-coated web passes through a drying oven 10 in    order to dry the adhesive, before being joined in the laminating nip    8 to the other web (the partially printed, partially demetallized    film) thereby making the final laminate which is wound up in    position 13.    Example of Demetallization Lacquer

The demetallization lacquer is generally a hard base such as NaOH or KOHdissolved in water or any other possible etching agent combined with afilm forming dispersion agent, also called encapsulating agent, such asnitro-cellulose encapsulating said hard base. The compatibility betweenthe etchant and dispersion agent is determinant. Other possibleadditives are usual processing additives such as anti foaming agents.

A series of demetallization lacquers are given in U.S. Pat. No.3,647,508 and can be adapted to the process of the present invention.

In summary, this invention has the following innovative aspects andadvantages:

-   -   the process achieves optimal clarity and transparency of the        demetallized area while eliminating the need for a washing step        previously considered necessary for such effect even when using        a demetallization lacquer designed to hold both the active agent        and its reaction product locked inside the multilayer structure.    -   printing, demetallization and laminating can be done in-line on        commonly available converting equipment, eliminating the need        for a dedicated demetallization line.    -   the in-line process, besides being much more efficient and        cost-effective, allows for more adequate quality control on the        final product allowing for adjustments in each of the previous        steps to be implemented immediately.    -   this process allows for in-line coating on the outside of the        laminate, e.g. a coldseal lacquer, in register with the printed        design.

Nomenclature

1-6: gravure stations 7: adhesive-coating station 8: laminating nip 9:gravure drying oven 10: adhesive drying oven 11: unwind film 1 12:unwind film 2 13: rewind laminate 20: film substrate layer 1 21:metallic layer 22: primer 23: printing ink 24: protective overlacquer25: demetallization lacquer 26: laminating adhesive 27: film substratelayer 2

1. A process for the partial demetallization of a multilayer laminatecomprising a first film and a second film wherein the first filmcomprises a first polymeric layer and a metallic layer and the secondfilm comprises a second polymeric layer and an adhesive layer, theprocess comprising applying an etchant lacquer to the metallic layer ofthe first polymeric film, applying an adhesive layer to the secondpolymeric film, and joining the first film and the second film whereinthe adhesive layer of the second film contacts the partiallydemetallized layer of the first film, wherein the lamination step isin-line with the demetallization step, wherein the etchant lacquercomprises at least one metal dissolving etchant on the metallic layer ina quantity of about the stoichiometrical amount needed to dissolve themetallic layer and to eliminate any chemical reactivity of the at leastone etchant towards the adhesive layer, wherein the dissolved metalremains within the multilayer laminate, and the dissolution of the metalcreates a substantially transparent window in the metallic layer in awashing-free step.
 2. The process of claim 1, wherein the process iscarried out on standard gravure or flexo printing presses or coatingequipment.
 3. The process of claim 1, further comprising a coatingoperation for treating the first film.
 4. The process of claim 1,further comprising a printing operation for treating the first film. 5.The process of claim 1, further comprising a coating operation and aprinting operation for treating the first film.
 6. The process of claim3, wherein the coating operation comprising a coating in register withthe demetallized area on a surface of the first polymeric layer that isdifferent than where the demetallization is carried out.
 7. The processof claim 4, wherein the printing operation comprises a patterned printin register with the demetallized area on a surface of the firstpolymeric layer that is different than where the demetallization iscarried out.
 8. The process of claim 2, wherein the amount the etchantlacquer is fine-tuned by choosing a suitable gravure cylinder depth. 9.The process of claim 1, wherein the amount the etchant lacquer isfine-tuned by adapting the concentration of the at least one etchant.10. The process of claim 2, wherein the amount of the etchant lacquer isfine-tuned by choosing a suitable gravure cylinder depth and by adaptingthe concentration of the at least one etchant.
 11. The process of claim1, wherein the demetallization step achieves a light transmission of atleast 90% within the demetallized area.
 12. The process of claim 1,wherein the concentration of the at least one etchant corresponds to aslight excess of the stoichiometrical amount needed to dissolve theamount of metal present on the multilayer laminate.
 13. A multilayerlaminate obtainable by the process of claim 1, comprising a window in asupported metallic layer wherein the window has the total quantity of aresidue resulting from the demetallization by means of the etchantlacquer.
 14. The process of claim 1 wherein the first polymeric layer isbiaxially oriented polypropylene.
 15. The process of claim 1 wherein thesecond polymeric layer is biaxially oriented polypropylene.
 16. Theprocess according to claim 1 wherein the first polymeric layer isbiaxially oriented polypropylene and the second polymeric layer isbiaxially oriented polypropylene.
 17. The process according to claim 1wherein the metallic layer is aluminum.
 18. The process according toclaim 16 wherein the metallic layer is aluminum.